Macrophages and granulocytes are involved in the inflammatory process. Proliferation and differentation of these cells from bone marrow hemopoietic stem cells (DMHSC) is regulated by the lymphokine colony-stimulating-factor (CSF). Because the percentage of stem cells in BM is very low, it is advantageous to use a dependent cell line to study how CSF regulates proliferation. Recently we described such a cell line, PT-18. In the present study we used this cell line to investigate the phase of the cell cycle in which CSF acts. Growth of cells was analyzed by staining them with propidium iodide and assaying DNA content by flow cytometry. Removal of CSF from the growth medium was followed by grandual accumulation of cells in the G1 phase; 18 hr later, about 85% of the cells were in the G1 phase. 10 hr after readdition of CSF, cells reentered the S phase. Furthermore, we found that CSF need not be present throughout the cell cycle; 6 hr of exposure of the cells cycle; 6 hr of exposure of the cells in G1 to CSF were sufficient to drive the cells back into cycle. Cells arrested in G1/S with aphidicolin and at G2/M with colcemid, and then released from the arrest, proceeded in the cycle in the absence of CSF, but arrested again in an early stage of the G1 phase. We conclude that CSF stimulates proliferation of cells by acting in the early G1 phase. CSF may act on BMHSC in a similar way, driving the cells from G0 into cycle.